Photodetector involving a MOSFET having a floating gate

ABSTRACT

A photodetector comprising a photoemissive surface capable of liberating photoelectrons. Photoelectrons are detected by a MOSFET having a floating gate, which is suitably charged before measurement in such a way that photoelectrons can cause a change in charge of the floating gate. The detected change indicates the amount of light received by the detector.

BACKGROUND

Measurement of weak light levels is a common procedure in science andtechnology.

One of the most sensitive photodetectors is a photomultiplier tube(PMT), or just photomultiplier. The basic structure of this device is avacuum tube containing a light sensitive photocathode and an electronmultiplier structure. Electric field by a high voltage is applied overthe system. Photons to be detected hit the photocathode from which theyliberate photoelectrons by a photoemission process. The electronmultiplier consists of a series (typically 6-16) of secondary emissionelectrodes, called dynodes, with rising electric potential arrangedbetween them. The photoelectrons from the cathode are directed to thefirst dynode where they produce several secondary electrons which are,in turn, directed to the next dynode where secondary emission isrepeated, and so on. This results in amplification so that the signalfrom the output electrode, anode, is high enough to be handledelectronically. Disadvantages of photomultiplier tubes are relativelyhigh cost and need for high voltage which limit and complicate theirversatility.

A different class are various semiconductor photodetectors, e.g.photodiodes, phototransistors and charge coupled devices (CCDs). Commonto them is that light is allowed to affect a semiconductor material,where it generates charge carriers (electrons and holes) that arecollected to produce an electrical signal. A problem with semiconductordetectors is that the carriers have to migrate in the bulk ofsemiconductor material where thermal energy produces a high backgroundnoise.

SUMMARY OF THE INVENTION

The present invention discloses a new type of photodetector which ischeap, sensitive and easy to construct. It comprises an evacuatedchamber containing a photoemissive surface capable of liberatingelectrons (photoelectrons) through photoelectric effect in response tolight photons. Characteristic to the present invention is that thephotoelectrons are detected by a metal oxide semiconductor type of fieldeffect transistor (MOSFET) having a floating gate, the gate beingsuitably charged before measurement. Photoelectron emission causes achange in gate charge, the change being indicative of the amount oflight received by the detector.

According to one embodiment, the photoemissive surface is unattached tothe gate, the latter being charged to a positive potential beforemeasurement. The positive charge attracts photoelectrons and directsthem to the gate where they neutralize its positive charge leading to adecrease in gate potential, the decrease being indicative of the amountof light received by the detector.

According to another embodiment, the photoemissive surface is processeddirectly on the floating gate which, in this case, is charged negativelybefore measurement. The liberated photoelectrons are collected to aseparate anode electrode or just to the metal wall of the device casing.This causes an increase in the gate potential, the increase beingindicative of the amount of light received by the detector.

During the photoelectron collection phase the presented photodetectordoes not require any electric power (voltage). Obviously, however, anadditional electric field can also be applied, in order to optimizephotoelectron collection.

OPERATION PRINCIPLE OF THE INVENTION

It is characteristic of the invention that electrons (photoelectrons)liberated from a photoemissive surface by a photoelectric effect inresponse to light photons are allowed to affect the surface of thefloating gate of a MOSFET (metal oxide semiconductor type of a fieldeffect transistor). The invention is based on measuring the effect ofphotoelectrons on the charge stored in the capacitance of the floatinggate of the MOSFET before measurement.

The photoelectrons are collected by means of the effect of the electricfield created by the gate after it has first been charged to a suitablepotential. This initial charging is accomplished, for example, byapplying the FN tunneling technique.

By measuring the conductivity of the drain-source channel of the MOSFET,the amount of gate charge can be determined without destroying thecharge itself. This is analogous to reading out the information storedin an analog EEPROM memory.

DRAWING

The drawing show diagrammatically two possible embodiments of theinvention.

FIG. 1 shows a preferred embodiment.

FIG. 2 shows an alternative embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows diagrammatically one embodiment for the photodetectorrelating to the invention. It should be noted that the elements offigure are not in scale. A photoemissive surface 20 receives lightphotons and liberates electrons (often called photoelectrons) through aphotoelectric effect. Photoemissive materials are previously known andcan be the same as those used in photocathodes of photomultipliers.

The photoelectrons are detected by the MOSFET 10. This device has threeelectrodes: source 11, drain 12 and gate 13. According to the presentinvention, the gate 13 is kept unconnected, that is, floating. Apositive charge is formed beforehand on the gate 13, for example, byapplying a sufficiently high voltage between the source 11 and the drain12. This causes the FN tunneling phenomenon to occur through the oxidelayer of the gate insulator 14, causing the potential in the floatinggate 13 to be set at the desired charge.

It is known that charge-retention properties of MOSFETs with floatinggates are excellent. They are, therefore, well suited for theconstruction of non-volatile memories, which include both digital andanalog EPROM and EEPROM memories. Previously, MOSFETs with chargedfloating gates have been used as detectors for ionizing radiation, asshown in PCT Publication WO 95/12134.

The positive charge creates an electric field which attractsphotoelectrons and directs them to the gate 13. On the surface of thegate 13 there is an uncovered area, or an area covered by a conductor,semiconductor or thin insulator. The thickness of the insulator may notexceed, for example, 1 mm to still enable passage of the electronsthrough it to the actual gate. Most preferably, however, a part of thesurface of the gate is completely uncovered. Accordingly, in the oxidelayer insulator 14 of the floating gate 13 a hole 17 has been formedthrough which the photoelectrons can directly reach the gate 13 surface.When hitting the gate 13, photoelectrons neutralize the positive chargethereon, causing a decrease in the gate 13 potential. The amount ofpotential decrease in a selected time interval is, therefore, indicativeof the amount of light received by the photodetector in that interval.

For proper operation the photoemissive surface 20 and the MOSFET 10 areenclosed in a casing 21 which is evacuated to confine a vacuum. Thecasing 21 has a transparent portion 22, e.g. of glass, through whichlight photons can reach the photoemissive surface 20. In the figurethere is shown a very useful structure where the photoemissive surface20 is processed on the interior surface of the transparent portion 22.The photoemissive surface 20 is connected to the metal wall of thecasing 21. Obviously, the photoemissive surface can also situate deeperin the interior cavity of the casing.

The gate 13 potential, being proportional to its charge, can bedetermined by measuring the conductivity of the source-drain channel ofthe MOSFET 10 without destroying the charge itself. Conductivity ismeasured by e.g. introducing a suitable voltage between source 11 anddrain 12 and by measuring the resulting source-drain current. In otherwords, the amount of light detected in a selected time interval can bedetermined by comparing the source-drain current after the detectioninterval to its initial value with the gate fully charged.

In order that a charging voltage could be applied between source 11 anddrain 12 and, correspondingly, that the change in the gate 13 potential(charge) could be measured as explained above, source 11 and drain 12are connected by means of conductors 26 and 27 to connectors 28 and 29,mounted in the wall of the casing 21.

According to another embodiment of the invention shown in FIG. 2 thephotoemissive surface 20 is processed directly on the gate 13 which, inthis case, is charged negatively before measurement. The liberatedphotoelectrons are collected to a separate anode 30 or just to the metalwall 21 of the casing. This causes an increase in the gate potential,the increase being indicative of the amount of light received by thedetector. This is determined by measuring conductivity of thesource-drain channel as above.

It is noteworthy that during the light detection (i.e. photoelectroncollection) phase, the presented photodetector does not require anyelectrical power (voltage). However, it is naturally possible to providean additional electric potential between the photoemissive surface andthe floating gate or between the photoemissive surface and the anode, inorder to enhance and optimize photoelectron collection.

We claim:
 1. A photodetector comprising:a photoemissive surface capableof emitting electrons in response to light photons to be detected, aMOSFET having a floating gate, said floating gate to be provided with acharge in such a way that emission of said electrons can cause a changein said charge, and a casing which encloses said photoemissive surfaceand said MOSFET, at least a portion of said casing being transparent tolight in such a way that said light can reach said photoemissivesurface.
 2. The photodetector according to claim 1, wherein saidphotoemissive surface is not in contact with said floating gate and saidcharge is positive.
 3. The photodetector according to claim 1, whereinsaid photoemissive surface is in contact with said floating gate andsaid charge is negative.
 4. The photodetector according to claim 3,further comprising a means for collecting said electrons.
 5. Aphotodetector comprising:a photoemissive surface capable of emittingelectrons in response to light photons to be detected, a MOSFET having afloating gate not in contact with said photoemissive surface, saidfloating gate to be provided with a positive charge, said floating gatethereby being capable of collecting said electrons, and a casing whichencloses said photoemissive surface and said MOSFET, at least a portionof said casing being transparent to light in such a way that said lightcan reach said photoemissive surface.
 6. The photodetector according toclaim 5, wherein said photoemissive surface is processed on the interiorsurface of said transparent portion of said casing.
 7. The photodetectoraccording to claim 5, further comprising a means for registering achange in said charge, the change being caused by said electrons, thechange thereby being indicative the amount of light received by thephotodetector.
 8. A photodetector comprising:a photoemissive surfacecapable of emitting electrons in response to light photons to bedetected, a MOSFET having a floating gate in contact with saidphotoemissive surface, said floating gate to be provided with a negativecharge, and a casing which encloses said photoemissive surface and saidMOSFET, at least a portion of said casing being transparent to light insuch a way that said light can reach said photoemissive surface.
 9. Thephotodetector according to claim 8, further comprising a means forregistering a change in said charge, the change being caused by saidelectrons, the change thereby being indicative to the amount of lightreceived by the photodetector.
 10. A method for detecting lightcomprising the steps of:providing a photodetector comprising: aphotoemissive surface capable of emitting electrons in response to lightphotons to be detected, a MOSFET having a floating gate, and a casingwhich encloses said photoemissive surface and said MOSFET, at least aportion of said casing being transparent to light in such a way thatsaid light can reach said photoemissive surface, charging said floatinggate to a preselected potential, allowing the light to be detected toaffect said photoemissive surface, thereby enabling electrons to beemitted from the photoemissive surface and causing a change in saidpreselected potential of said floating gate, and after a selected timeregistering the change in said preselected potential, said change beingindicative of the amount of light received by said photodetector. 11.The method for detecting light according to claim 10, wherein the stepof charging is performed by applying a voltage between the sourceelectrode and the drain electrode of said MOSFET.
 12. A method fordetecting light comprising the steps of:providing a photodetectorcomprising: a photoemissive surface capable of emitting electrons inresponse to light photons to be detected, a MOSFET having a floatinggate, said floating gate having a charge thereon, and a casing whichencloses said photoemissive surface and said MOSFET, at least a portionof said casing being transparent to light in such a way that said lightcan reach said photoemissive surface, allowing the light to be detectedto affect said photoemissive surface, thereby enabling the electronsemitted from the photoemissive surface to cause a change in said charge,and after a selected time registering the change in said charge, saidchange being indicative of the amount of light received by saidphotodetector.